Aluminum alloy forging and production method thereof

ABSTRACT

Provided are an Al—Mg—Si based aluminum alloy forging excellent in mechanical properties in room temperature and hardly causing recrystallized grains and a production method thereof. An aluminum alloy forging consists of: Cu: 0.15 mass % to 1.0 mass %, Mg: 0.6 mass % to 1.15 mass %; Si: 0.95 mass % to 1.25 mass %; Mn: 0.4 mass % to 0.6 mass %; Fe: 0.2 mass % to 0.3 mass %; Cr: 0.11 mass % to 0.25 mass %; Ti: 0.012 mass % to 0.035 mass %; B: 0.0001 mass % to 0.03 mass %; Zn: 0.25 mass % or less; Zr: 0.05 mass % or less; and the balance being aluminum and inevitable impurities. The number of intermetallic compounds of Mg 2 Si with a minor axis of 0.5 μm or more present in a visual field area of 1.5815 mm 2  is 100 or less when a sectional structure of the aluminum alloy forging is observed at a magnification of 1,000 times.

BACKGROUND OF THE INVENTION Technical Field

The present invention relates to an Al—Mg—Si based aluminum alloy forging excellent in mechanical properties and a production method thereof.

Note that in this specification, the term “FE-SEM” means a field emission scanning electron microscope.

Description of the Related Art

The following description sets forth the inventor's knowledge of the related art and problems therein and should not be construed as an admission of knowledge in the prior art.

In recent years, an aluminum alloy has been expanding its application as a structural member for various products by taking advantage of its lightness. For example, high tensile strength steel has been used for undercarriage members and bumper components of automobiles. However, in recent years, high strength aluminum alloy materials have been used. Automobile components, such as, e.g., suspension components, were mostly made of ferrous materials, but are often replaced with aluminum materials or aluminum alloy materials for weight reduction as a primary object.

These automobile components are required to have excellent corrosion resistance, high strength, and excellent workability, so an Al—Mg—Si based alloy, especially A6061 aluminum alloy, is widely used as an aluminum alloy material. To improve the strength of such automobile components, automobile components are manufactured by performing a forging process, which is one of plastic working, using an aluminum alloy material as a blank material to be processed. In recent years, to cope with cost reduction demands, a suspension component obtained by forging a cast member as a blanc material to be processed without performing extrusion and then subjecting it to a T6 treatment has begun to be put into practice. For further weight reduction, a high strength alloy is being developed to replace the conventional A6061 aluminum alloy (see Patent Documents 1 to 3 below).

PRIOR ART DOCUMENTS

-   Patent Document 1: Japanese Unexamined Patent Application     Publication No. H5-59477 -   Patent Document 2: Japanese Unexamined Patent Application     Publication No. H5-247574 -   Patent Document 3: Japanese Unexamined Patent Application     Publication No. H6-256880

Problems to be Solved by the Invention

However, in an Al—Mg—Si based high strength alloy described above, the processing structure is recrystallized during the forging and heat treatment steps, causing coarse crystal grains, which makes it impossible to obtain sufficiently high strength. Therefore, in order to prevent the generation of coarse recrystallized grains, there are proposals in which Zr is added to prevent the generation of recrystallized grains (Patent Documents 1 and 2 described above).

However, although the addition of Zr is effective for preventing recrystallization, there are the following problems.

(1) The addition of Zr weakens the crystal grain refinement effect of the Al—Ti—B based alloy and coarsens the crystal grain of the casting itself, thereby leading to a decrease in strength of the workpiece (forging) after plastic working.

(2) The crystal grain refinement effect of the casting itself is weakened, thereby likely causing cracking of a casting, which increases the internal defects. As a result, the yield deteriorates.

(3) Zr forms compounds with an Al—Ti—B based alloy. The compounds deposit on the bottom of the furnace for storing a molten alloy, which contaminants the furnace. In the produced casting, the compounds coarsely crystallize in the produced casting, thereby reducing the strength.

As described above, although the addition of Zr is effective for preventing recrystallization, there is a problem that it is difficult to maintain strength stability.

Preferred embodiments of the present invention have been made in view of the above-described and/or other problems in the related art. The preferred embodiments of the present invention can significantly improve existing methods and/or equipment.

SUMMARY OF THE INVENTION

The present invention has been made in view of the above-described technical background, and an object thereof is to provide an Al—Mg—Si based aluminum alloy forging excellent in mechanical properties in room temperature and hardly causing recrystallized grains and to provide a production method of the Al—Mg—Si based aluminum alloy forging.

Other objects and benefits of the present invention will be apparent from the following preferred embodiments.

Means for Solving the Problem

In order to achieve the above-described object, the present invention provides the following means.

[1] An aluminum alloy forging consists of:

Cu: 0.15 mass % to 1.0 mass %; Mg: 0.6 mass % to 1.15 mass %; Si: 0.95 mass % to 1.25 mass %; Mn: 0.4 mass % to 0.6 mass %; Fe: 0.2 mass % to 0.3 mass %; Cr: 0.11 mass % to 0.25 mass %; Ti: 0.012 mass % to 0.035 mass %; B: 0.0001 mass % to 0.03 mass %; Zn: 0.25 mass % or less; Zr: 0.05 mass % or less; and the balance being aluminum and inevitable impurities. The number of intermetallic compounds of Mg₂Si with a minor axis of 0.5 μm or more present in a visual field area of 1.5815 mm² is 100 or less when a sectional structure of the aluminum alloy forging is observed at a magnification of 1,000 times.

[2] A method of producing an aluminum alloy forging, the aluminum alloy forging being recited in the aforementioned Item [1], the method includes:

a molten metal forming process of obtaining a molten metal;

a casting process of casting the molten metal obtained in the molten metal forming process to obtain a casting;

a homogenization heat treatment process of subjecting the casting obtained in the casting process to a homogenization heat treatment;

a forging process of forging the casting after the homogenization heat treatment process to obtain a forging;

a solution heat treatment process of subjecting the forging obtained in the forging process to a solution heat treatment;

a quenching process of quenching the forging after the solution heat treatment process; and

an aging process of aging the forging after the quenching process.

[3] The method of producing an aluminum alloy forging as recited in the aforementioned Item [2],

wherein the homogenization heat treatment process is performed by holding the casting obtained in the casting process at a temperature of 370° C. to 560° C. for 4 hours to 10 hours;

wherein the forging process is performed by forging the casting after the homogenization heat treatment process at a heating temperature of 450° C. to 560° C.;

wherein the solution treatment process is performed by raising a temperature of the forging obtained in the forging process at a temperature raising rate of 5.0° C./min or more from 20° C. to 500° C. and holding the forging at 530° C. to 560° C. for 0.3 hours to 3 hours,

wherein the quenching process is performed by bringing an entire surface of the forging into contact with quenching water in a water tank within 5 seconds to 60 seconds after the solution treatment process for more than 5 minutes but not exceeding 40 minutes, and

wherein the aging process is performed by heating the forging after the quenching process at a temperature of 180° C. to 220° C. for 0.5 hours to 8 hours.

Effects of the Invention

According to the invention recited in the above-described Item [1], in addition to the limitation of the alloy composition, the number of intermetallic compounds of Mg₂Si with a minor axis of 0.5 μm or more present in a visual field area of 1.5815 mm² is 100 or less when a sectional structure of the aluminum alloy forging is observed at a magnification of 1,000 times. With this configuration, it is possible to obtain an Al—Mg—Si based aluminum alloy forging excellent in mechanical properties in room temperature and hardly causing recrystallized grains.

According to the invention recited in the above-described Item [2], it is possible to provide a production method of an Al—Mg—Si based aluminum alloy forging excellent in mechanical properties in room temperature and hardly causing recrystallized grains.

According to the invention recited in the above-described Item [3], it is possible to provide a production method of an Al—Mg—Si based aluminum alloy forging further excellent in mechanical properties in room temperature and hardly causing recrystallized grains.

BRIEF DESCRIPTION OF THE DRAWINGS

The preferred embodiments of the present invention are shown by way of example, and not limitation, in the accompanying figures.

FIG. 1 is a perspective view illustrating an example of an aluminum alloy forging according to the present invention.

FIG. 2 is an SEM photograph showing the Mg₂Si structure in which the sectional structure of the aluminum alloy forging in Example 1 is imaged with an FE-SEM (field emission scanning electron microscope).

EMBODIMENTS FOR CARRYING OUT THE INVENTION

FIG. 1 is a perspective view showing an example of an aluminum alloy forging of the present invention. FIG. 2 is an SEM photograph showing an Mg₂Si structure in which a sectional structure of an aluminum alloy forging in Example 1 is imaged with an FE-SEM (field emission scanning electron microscope).

The aluminum alloy forging and its production method according to the present invention will be described in detail. Note that the following embodiments are merely illustrative, and the present invention is not limited to these embodiments and can be appropriately modified without departing from the scope of the technical concept of the present invention.

The aluminum alloy forging of the present invention consists of: Cu: 0.15 mass % to 1.0 mass %; Mg: 0.6 mass % to 1.15 mass %; Si: 0.95 mass % to 1.25 mass %; Mn: 0.4 mass % to 0.6 mass %; Fe: 0.2 mass % to 0.3 mass %; Cr: 0.11 mass % to 0.25 mass %; Ti: 0.012 mass % to 0.035 mass %; B: 0.0001 mass % to 0.03 mass %; Zn: 0.25 mass % or less; Zr: 0.05 mass % or less; and the balance being aluminum and inevitable impurities, wherein the number of intermetallic compounds of Mg₂Si with a minor axis of 0.5 μm or more present in a visual field area of 1.5815 mm² is 100 or less when a sectional structure of the aluminum alloy forging is observed at a magnification of 1,000 times.

Mg₂Si in the present invention is an intermetallic compound. As shown in FIG. 2, it precipitates in the structure of the aluminum alloy forging, which contributes to the strength improvement of the aluminum alloy forging.

As shown in FIG. 2, in the aluminum alloy forging 1 according to the embodiment, the number of intermetallic compounds of Mg₂Si with a minor axis of 0.5 μm or more present in a visual field area of 1.5815 mm² is 100 or less when a sectional structure of the aluminum alloy forging 1 is observed using an FE-SEM at a magnification of 1,000 times.

In this embodiment, the sectional structure of the aluminum alloy forging 1 is observed using an FE-SEM. However, other electron microscopes other than the FE-SEM may be used. Further, the observation of the sectional structure of the aluminum alloy forging 1 is not limited to an observation using an electronic microscope. The observation may be performed by any method as long as it is possible to observe at a magnification of 1,000 times.

According to the aluminum alloy forging of the present invention, in addition to the alloy composition described above, the number of intermetallic compounds of Mg₂Si with a minor axis of 0.5 μm or more present in a visual field area of 1.5815 mm² is 100 or less when a sectional structure of the aluminum alloy forging is observed at a magnification of 1,000 times. Therefore, it is possible to obtain an Al—Mg—Si based aluminum alloy forging excellent in mechanical properties and hardly causing recrystallized grains.

Next, the production method of the Al—Mg—Si based aluminum alloy forging according to the present invention will be described.

The production method of the aluminum alloy forging according to the present invention includes a molten metal forming process, a casting process, a homogenization heat treatment process, a forging process, a solution heat treatment process, a quenching process, and an aging process.

(Molten Metal Forming Process)

In the molten metal forming process, a molten aluminum alloy is obtained (prepared). The molten aluminum alloy consists of: Cu: 0.15 mass % to 1.0 mass %; Mg: 0.6 mass % to 1.15 mass %; Si: 0.95 mass % to 1.25 mass %; Mn: 0.4 mass % to 0.6 mass %; Fe: 0.2 mass % to 0.3 mass %; Cr: 0.11 mass % to 0.25 mass %; Ti: 0.012 mass % to 0.035 mass %; B: 0.0001 mass % to 0.03 mass %; Zn: 0.25 mass % or less; Zr: 0.05 mass % or less; and the balance being aluminum and inevitable impurities. In the aluminum alloy, the Zn content may be 0% (Zn-free), and the Zr content rate may be 0% (Zr-free).

(Casting Process)

A casting is obtained by subjecting the obtained molten aluminum alloy to a casting process. The continuous casting method of obtaining the casting is not specifically limited, and various known continuous casting methods (e.g., a vertical type continuous casting method, a horizontal type continuous casting method) can be exemplified. As a vertical type continuous casting method, a hot-top casting method or the like is used.

Hereinafter, as an example of a continuous casting method, a brief description will be directed to the case in which an aluminum alloy continuously cast material is produced by a hot-top casting method using a hot-top casting apparatus (i.e., the case in which an aluminum alloy continuously cast material is produced by subjecting a molten aluminum alloy to continuous casting by a hot-top casting method).

A hot-top casting apparatus is equipped with a mold (casting die), a molten metal receptor (header), etc. The mold is cooled by cooling water filled therein. The receptor is generally made of a fireproof material and is arranged above the mold. The molten aluminum alloy in the receptor is poured down into a cooled mold to be cooled and solidified at a predetermined cooling rate by the cooling water spouted from the mold, and immersed in water (its temperature: about 20° C.) in a water tank to be completely solidified. With this, a long continuously cast material having a rod shape or the like is obtained.

(Homogenization Heat Treatment Process)

The obtained casting is subjected to a homogenization heat treatment for 4 hours to 10 hours at a temperature between 370° C. and 560° C. By the homogenization heat treatment in this temperature range, the casting is sufficiently homogenized, and solute atoms are adequately incorporated. Therefore, sufficient strength can be obtained by the subsequent aging process.

(Forging Process)

In the forging process, the obtained casting is forged at a heating temperature of 450° C. to 560° C. to obtain a forging (e.g., an automobile suspension arm component). At this time, the starting temperature of forging a forging blank is set to 450° C. to 560° C. When the starting temperature is lower than 450° C., the deformation resistance increases, which prevents sufficient processing. On the other hand, when it exceeds 560° C., defects, such as, e.g., forging cracks and eutectic melting, are likely to occur.

(Solution Treatment Process)

A solution treatment process is a process for reducing distortion caused in the forging process and performing a solid solution of solute elements. In this solution heat treatment process, a solution heat treatment is performed on the forging by raising a temperature of the forging obtained in the forging process at a temperature raising rate of 5.0° C./min or more from 20° C. to 500° C. and heating the forging at 530° C. to 560° C. for 0.3 hours to 3 hours. When the treatment temperature is lower than 530° C., the solution heat treatment does not proceed, which cannot realize an increase in strength by age precipitation. When the treatment temperature exceeds 560° C., although a solid solution of solute elements is further promoted, eutectic melting and recrystallization are likely to occur.

(Quenching Process)

A quenching process is a heat treatment for quickly cooling down the solid solution state obtained by the solution heat treatment to thereby form a supersaturated solid solution. The quenching process is performed by bringing an entire surface of the forging into contact with quenching water in a water tank within 5 seconds to 60 seconds after the solution treatment process for more than 5 minutes but not exceeding 40 minutes.

(Aging Process)

The forging subjected to the quenching process is heated for 0.5 hours to 8 hours at a temperature of 180° C. to 220° C. to perform an aging treatment. When the processing temperature is less than 180° C. or the processing time is less than 0.5 hours, Mg₂Si based precipitates for improving tensile strength cannot sufficiently grow. In contrast, when the processing temperature exceeds 220° C., Mg₂Si based precipitates become too coarse to sufficiently improve the tensile strength.

As described above, according to the production method of the aluminum alloy forging of the present invention, it is possible to provide a production method of an Al—Mg—Si based aluminum alloy forging excellent in mechanical properties and hardly causing recrystallized grains.

Next, the composition of the “aluminum alloy” in the aluminum alloy forging and the production method thereof according to the above-described present invention will be described in detail below. The aluminum alloy consists of: Cu: 0.15 mass % to 1.0 mass %; Mg: 0.6 mass % to 1.15 mass %; Si: 0.95 mass % to 1.25 mass %; Mn: 0.4 mass % to 0.6 mass %; Fe: 0.2 mass % to 0.3 mass %; Cr: 0.11 mass % to 0.25 mass %; Ti: 0.012 mass % to 0.035 mass %; B: 0.0001 mass % to 0.03 mass %; Zn: 0.25 mass % or less; Zr: 0.05 mass % or less, and the balance being aluminum and inevitable impurities.

Si co-exists with Mg to form Mg₂Si based precipitates, which contributes to an improvement in the strength of a finished product. To further enhance the strength of the finished product after the aging treatment by excessively adding Si to exceed the amount of generating Mg₂Si with respect to the amount of Mg, which will be discussed later, the Si content is preferably 0.95 mass % or more. On the other hand, when the Si contents exceed 1.25 mass %, the grain boundary precipitation of Si increases, which easily causes grain boundary embrittlement. This deteriorates the plastic workability of the casting and reduces the toughness of the finished product. Further, there is a possibility that the average particle diameter of the crystallized substance of the casting exceeds a predetermined upper limit. Therefore, the Si content is required to fall within the range of 0.95 mass % to 1.25 mass %.

Mg co-exists with Si to form Mg₂Si based precipitates, which contributes to the improvement in the strength of the finished product. When the Mg content is less than 0.6 mass %, there is a possibility that precipitation strengthening becomes less effective. On the other hand, when it exceeds 1.15 mass %, the plastic workability of the casting and the toughness of the finished product deteriorates. Further, there is a possibility that the average particle diameter of the crystallized substance of the casting exceeds a predetermined upper limit. Therefore, the Mg content is required to fall within the range of 0.6 mass % to 1.15 mass %.

Cu increases the amount of apparent supersaturation of Mg₂Si based precipitates and increases the amount of precipitation of Mg₂Si, which significantly facilitates the age-hardening of the finished product. When the Cu content is less than 0.15 mass %, the Q phase (Al—Cu—Mg—Si) effective for strengthening precipitation is hard to be generated, resulting in deteriorated mechanical properties. On the other hand, when the Cu content exceeds 1.0 mass %, the forging processability of the casting and the toughness of the finished product decrease, which may further significantly decrease the corrosion resistance. Therefore, the Cu content is required to fall within the range of 0.15 mass % to 1.0 mass %.

Mn crystallizes as an AlMnSi phase, and non-crystallized Mn precipitates to suppress recrystallization. This recrystallization suppressing effect makes the crystal grain finer even after the plastic working, resulting in improved toughness and corrosion resistance of the finished product. When the Mn content is less than 0.4 mass %, the above-described effects may be reduced. On the other hand, when it exceeds 0.6 mass %, huge intermetallic compounds may be generated, and therefore the casting structure of the present invention may not be satisfied. Therefore, the Mn content is required to fall within the range of 0.4 mass % to 0.6 mass %.

Cr also crystallizes as an AlCrSi phase, and non-crystallized Cr precipitates to suppress recrystallization. This recrystallization suppressing effect makes the crystal grain finer even after the plastic working, which improves the toughness and the corrosion resistance of the finished product. When the Cr content is less than 0.11 mass %, the above-described effect may be reduced. On the other hand, when it exceeds 0.25 mass %, a huge intermetallic compound occurs, and therefore the casting structure of the present invention may not be satisfied. Therefore, the Cr content is required to fall within the range of 0.11 mass % to 0.25 mass %.

Fe bonds to Al and Si in the alloy and is crystallized, which prevents crystal grain coarsening. When the Fe content is less than 0.2 mass %, the above-described effect may not be obtained. On the other hand, when it exceeds 0.3 mass %, rough intermetallic compounds will be generated. Therefore, there is a possibility that the plastic workability deteriorates. Therefore, the Fe content is required to fall within the range of 0.2 mass % to 0.3 mass %.

Zn is treated as impurities. When it exceeds 0.25 mass %, the corrosion of aluminum itself is accelerated, which deteriorates the corrosion resistance. Therefore, the Zn content is required to be set to less than 0.25 mass %.

Zr is treated as impurities. When it exceeds 0.05 mass %, the crystal grain refinement effect of the Al—Ti—B based alloy decreases, causing a decrease in the strength of the workpiece after the plastic working. Therefore, the Zr content is required to be set to 0.05 mass % or less.

Ti is an alloy element useful for miniaturization of crystal grains and prevents the generation of cracking of a casting, etc., in a continuously cast bar. When the Ti content is less than 0.012 mass %, the miniaturization effect cannot be obtained. On the other hand, when the Ti content exceeds 0.035 mass %, there is a possibility that rough Ti compounds will be crystallized, resulting in deterioration of the toughness. Therefore, the Ti content is required to fall within the range of 0.012 mass % to 0.035 mass %.

Like Ti, B is an element useful for the crystal grain refinement. When the B content is less than 0.0001 mass %, the effect cannot be obtained. On the other hand, when it exceeds 0.03 mass %, there is a possibility that the toughness deteriorates. Thus, the B content is required to fall within the range of 0.0001 mass % to 0.03 mass %.

EXAMPLES

Next, specific examples of the present invention will be described. However, it should be noted that the present invention is not particularly limited to these examples.

Examples 1 to 12

A circular cross-sectional continuously cast material having a diameter of 54 mm was prepared by the aluminum alloy of the alloy composition shown in Table 1. The continuously cast material was subjected to a homogenization heat treatment under the conditions shown in Table 1 and then air-cooled. The obtained cast material was subjected to a forging process under the conditions shown in Table 1 to be plastically deformed into a configuration of an automobile suspension arm component shown in FIG. 1. Next, after performing a solution heat treatment under the conditions shown in Table 1, a quenching process was performed under the conditions shown in Table 1, and then an aging treatment was conducted under the conditions shown in Table 1 to obtain an aluminum alloy forging 1.

Comparative Examples 1 to 10

A circular cross-sectional continuously cast material having a diameter of 54 mm was prepared by the aluminum alloy of the alloy composition shown in Table 2. The continuously cast material was subjected to a homogenization heat treatment under the conditions shown in Table 2 and then air-cooled. The obtained cast material was subjected to a forging process under the conditions shown in Table 2 to be plastically deformed into a configuration of an automobile suspension arm component shown in FIG. 1. Next, after performing a solution heat treatment under the conditions shown in Table 2, a quenching process was performed under the conditions shown in Table 2, and then an aging treatment was conducted under the conditions shown in Table 2 to obtain an aluminum alloy forging 1.

TABLE 1 Ex. 1 Ex. 2 Ex. 3 Ex. 4 Ex. 5 Ex. 6 Ex. 7 Ex. 8 Ex. 9 Ex. 10 Ex. 11 Ex. 12 Compo- Cu (mass %)  0.4 0.4 0.4 0.4 0.35 0.35 0.35 0.35 0.35 0.35 0.35 0.35 sition Mg (mass %)  0.84 0.84 0.84 0.84 0.87 0.87 0.87 0.87 0.87 0.87 0.87 0.87 Si (mass %) 1.08 1.08 1.08 1.08 1.05 1.05 1.05 1.05 1.05 1.05 1.05 1.05 Mn (mass %)  0.49 0.49 0.49 0.49 0.5 0.5 0.5 0.5 0.5 0.5 0.5 0.5 Fe (mass %) 0.25 0.25 0.25 0.25 0.25 0.25 0.27 0.27 0.27 0.27 0.27 0.27 Cr (mass %) 0.15 0.15 0.15 0.15 0.17 0.17 0.17 0.17 0.17 0.17 0.17 0.17 Ti (mass %) 0.02 0.02 0.02 0.02 0.02 0.02 0.02 0.02 0.02 0.02 0.02 0.02  S (mass %) 0.004 0.004 0.004 0.004 0.005 0.005 0.005 0.005 0.005 0.005 0.005 0.005 Condi- Homoge- Temp. 500 500 500 420 470 470 470 470 470 470 470 470 tions/ nization Holding 7 7 7 7 7 7 7 7 7 7 7 7 Eval- treatment time uation process Forging Temp. 500 500 500 500 500 500 500 500 500 500 500 500 process Solution Temp. 22.5 22.5 22.5 8.75 240 240 240 240 180 180 22.5 22.5 treatment raising process rate [° C./min] Temp. [° C.] 530 530 530 530 540 540 540 540 540 540 540 540 Holding 180 180 180 180 30 180 30 180 30 180 30 180   time [min] Quenching Time until 60 15 5 60 15 15 15 15 15 5 15 5 process submersion [s] Temp. [° C.] 60 60 60 60 60 60 60 60 60 60 60 60 Submerged 7 7 10 7 7 10 15 15 10 7 10 7  Time [min] Aging Temp. [° C.] 200 200 200 210 200 200 210 210 200 200 210 210 process  Time [min] 30 60 90 30 60 60 45 45 60 60 45 45 The number of Mg₂Si 77 77 78 75 60 60 65 63 60 60 65 63 Tensile strength at 376 381 383 368 401 400 392 392 389 390 379 378 room temperature Overall evaluation ○ ○ ○ ○ ⊚ ⊚ ⊚ ⊚ ⊚ ⊚ ○ ○

TABLE 2 Comp. Comp. Comp. Comp. Comp. Comp. Comp. Comp. Comp. Comp. Ex. 1 Ex. 2 Ex. 3 Ex. 4 Ex. 5 Ex. 6 Ex. 7 Ex. 8 Ex. 9 Ex. 10 Compo- Cu (mass %)  0.4 0.34 0.4 0.26 0.26 0.34 0.38 0.38 0.3 0.4 sition Mg (mass %)  0.84 0.84 0.84 1.07 1.07 0.84 0.87 0.87 0.65 1 Si (mass %) 1.08 0.95 1.08 0.68 0.68 0.95 0.96 0.96 0.9 0.8 Mn (mass %)  0.49 0.3 0.49 0.01 0.01 0.3 0.3 0.3 0.2 0.2 Fe (mass %) 0.25 0.27 0.25 0.26 0.26 0.27 0.26 0.26 0.26 0.26 Cr (mass %) 0.15 0.12 0.15 0.12 0.12 0.12 0.12 0.12 0.2 0.2 Ti (mass %) 0.02 0.03 0.02 0.02 0.02 0.03 0.016 0.016 0.019 0.018  S (mass %) 0.004 0.004 0.004 0.003 0.003 0.004 0.004 0.004 0.003 0.003 Condi- Homoge- Temp. 500 560 500 560 560 500 500 560 500 500 tions/ nization Holding 7 9 7 7 9 7 8 8 7 7 Eval- treatment time uations process Forging Temp. 500 560 500 560 560 560 500 560 500 500 process Solution Temp. 1 .00 2.67 2 .67 1.00 1.09 1.33 1.09 2. 67 2 .67 2.67 treatment raising process rate [° C./min] Temp. [° C.] 530 500 530 500 560 500 500 500 500 500 Holding 180 180 180 180 180 180 180 180 3 3   time [min] Quenching Time until 15 15 15 15 15 15 15 90 90 15 process submersion [s] Temp. [° C.] 60 60 60 60 60 60 60 60 60 60 Submerged 0.5 1 10 0.5 3 7 10 7 10 3  Time [min] Aging Temp. [° C.] 200 180 200 180 180 180 180 180 180 180 process  Time [min] 0 240 180 240 240 240 360 360 240 360 The numbcr of Mg₂Si 180 162 105 165 70 164 172 79 160 155 Tensile strength at 261 336 349 289 333 341 338 340 343 339 room temperature Overall evaluation × × Δ × × × × × × ×

Aluminum forgings obtained as described above were evaluated based on the following evaluation method. The results are shown in Tables 1 and 2.

Note that “the number of Mg₂Si” in Tables 1 and 2 is the number of Mg₂Si present in a matrix of each aluminum alloy forging. A sample piece for metal structure observation having a size of approximately 10 mm length×10 mm width×10 mm depth was cut out of the center of the thickest part of the obtained aluminum alloy forging, and this sample was polished using a cross-section polisher. Then, as shown in FIG. 2, an FE-SEM photograph (field emission scanning electron microscopic photograph, magnification: 1,000 times) of the sample piece after the polishing was taken. The number of Mg₂Si present in the range of the visual field area of 1.5815 mm² in the SEM photograph and having a minor axis of 0.5 μm or more was obtained (evaluated).

<Tensile Strength Evaluation Method at Room Temperature>

From the obtained aluminum alloy forging, a tensile test piece having a gauge distance of 25.4 mm, a parallel portion diameter of 6.4 mm was taken and subjected to a tensile test at room temperature (25° C.) to measure the room temperature tensile strength. And it was evaluated based on the following criteria.

(Criteria)

“⊚”: Room temperature tensile strength was 385 MPa or more. “◯”: Room temperature tensile strength was 365 MPa or more and less than 385 MPa “Δ”: Room temperature tensile strength was 345 MPa or more and less than 365 MPa “X”: Room temperature tensile strength was less than 345 MPa.

As obvious from the Tables, the aluminum alloy forgings of Examples 1 to 12 produced by the production method of the present invention were superior in tensile strength at room temperature and had 100 or less of Mg₂Si.

In contrast, the aluminum alloy forgings of Comparative Examples 1 to 10, which deviated from the specified scope of the present invention, were inferior in tensile strength at room temperature.

INDUSTRIAL APPLICABILITY

The forging obtained by the production method of the aluminum alloy forging according to the present invention is excellent in the mechanical strength at room temperature, and therefore is preferably used for an automobile undercarriage member, such as, e.g., suspension arm components, but not limited to such applications.

This application claims the priority to Japanese Patent Application No. 2019-196211, which was filed on Oct. 29, 2019, the contents of which are incorporated herein by reference in their entirety.

The terms and expressions used herein are used for the purpose of explanation and are not intended to be used solely for interpretation, and should not be recognized as excluding any equivalents of the features presented and described herein, and should be recognized as allowing various variations within the claimed range of the present invention.

DESCRIPTION OF SYMBOLS

-   1: Aluminum alloy forging 

1. An aluminum alloy forging consisting of: Cu: 0.15 mass % to 1.0 mass %; Mg: 0.6 mass % to 1.15 mass %; Si: 0.95 mass % to 1.25 mass %; Mn: 0.4 mass % to 0.6 mass %; Fe: 0.2 mass % to 0.3 mass %; Cr: 0.11 mass % to 0.25 mass %; Ti: 0.012 mass % to 0.035 mass %; B: 0.0001 mass % to 0.03 mass %; Zn: 0.25 mass % or less; Zr: 0.05 mass % or less; and the balance being aluminum and inevitable impurities, wherein the number of intermetallic compounds of Mg₂Si with a minor axis of 0.5 μm or more present in a visual field area of 1.5815 mm² is 100 or less when a sectional structure of the aluminum alloy forging is observed at a magnification of 1,000 times.
 2. A method of producing an aluminum alloy forging, the aluminum alloy forging being recited in claim 1, the method comprising: a molten metal forming process of obtaining a molten metal; a casting process of casting the molten metal obtained in the molten metal forming process to obtain a casting; a homogenization heat treatment process of subjecting the casting obtained in the casting process to a homogenization heat treatment; a forging process of forging the casting after the homogenization heat treatment process to obtain a forging; a solution heat treatment process of subjecting the forging obtained in the forging process to a solution heat treatment; a quenching process of quenching the forging after the solution heat treatment process; and an aging process of aging the forging after the quenching process.
 3. The method of producing an aluminum alloy forging, as recited in claim 2, wherein the homogenization heat treatment process is performed by holding the casting obtained in the casting process at a temperature of 370° C. to 560° C. for 4 hours to 10 hours; wherein the forging process is performed by forging the casting after the homogenization heat treatment process at a heating temperature of 450° C. to 560° C.; wherein the solution treatment process is performed by raising a temperature of the forging obtained in the forging process at a temperature raising rate of 5.0° C./min or more from 20° C. to 500° C. and holding the forging at 530° C. to 560° C. for 0.3 hours to 3 hours, wherein the quenching process is performed by bringing an entire surface of the forging into contact with quenching water in a water tank within 5 seconds to 60 seconds after the solution treatment process for more than 5 minutes but not exceeding 40 minutes, and wherein the aging process is performed by heating the forging after the quenching process at a temperature of 180° C. to 220° C. for 0.5 hours to 8 hours. 